The invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of picture elements arranged in rows and columns, each picture element being constituted by two picture electrodes arranged on the facing surfaces of the supporting plates, a system of row and column electrodes for driving the picture elements, the row electrodes being arranged on the one supporting plate and the column electrodes being arranged on the other supporting plate, and a system of switching elements in which a switching unit including at least one Schottky diode as a switching element is arranged in series with each picture element between a row and a column electrode.
In this respect it is to be noted that the terms row electrode and column electrode in this Application may be interchanged, if desired, so that a column electrode can be a row electrode while simultaneously a row electrode can be a column electrode.
A display device of this type is suitable for the display of alphanumerical and video information by means of passive electro-optical display media such as liquid crystals, electrophoretic suspensions and electrochromic materials.
The known passive electro-optical display media generally have an insufficiently steep transition characteristic with respect to the applied voltage and/or they have an insufficient intrinsic memory. As a result of these properties the number of lines to be driven is small in multiplexed matrix display devices. Owing to the lack of memory the information presented to a selected row electrode via the column electrodes must be written over and over again. Moreover, the voltages presented to the column electrodes are not only present across the picture elements of a driven row electrode but also across the picture electrodes of all other rows. Consequently, picture elements undergo an effective voltage during the period when they are not driven, which voltage may be sufficient to bring a picture element to the on-state. With an increasing number of row electrodes the ratio of the effective voltage undergone by a picture element in the on or the off-state decreases. The lack of a sufficiently steep transition characteristic consequently causes the contrast between picture elements in the on and off-states to decrease.
It is known that the number of rows to be driven may be increased by providing an extra switch for each picture element. This switch provides for a sufficient threshold with respect to the applied voltage and it ensures that the information presented to a driven row electrode remains available across a picture element during the period when the other row electrodes are driven. The switch also prevents a picture element from undergoing an effective voltage during the period when it is not driven.
A display device of the type mentioned in the opening paraagraph is described in British Patent GB 2,129,183 A. This patent describes the use of Schottky diodes as switches. These have various advantages over semiconductor pn diodes, such as a higher current density, more rapid switching times and smaller crosstalk, which results in a high resolution of the display device. The Schottky diodes referred to in this patent have a vertical structure comprising a layer of amorphous silicon between two metal layers constituting a Schottky contact (anode) and an ohmic contact (cathode), respectively, where the ohmic contact is a highly doped silicon layer.
A drawback of such a Schottky diode is that the layer of amorphous silicon has a high resistivity. This resistivity cannot be decreased by doping the semiconductor material because the cathode contact then no longer functions as a Schottky contact. Moreover, a second drawback is the presence of so-called "pin holes" which in the amorphous silicon layer, may be filled up by a metal contact layer and may thus cause short circuits between cathode and anode. This risk will be considerably greater if the semiconductor layer is made thinner in order to decrease the resistance between cathode and anode.
Other drawbacks are that such diodes cover a relatively large surface area due to alignment tolerances. Consequently, they have a high capacitance which increases the crosstalk to a considerable extent. Leakage currents also often occur along the periphery, which necessitates an additional passivation step.